In the continuous casting of steel, molten metal is cast directly into thin strip by a casting machine. The shape of the strip is determined by the mold of the casting machine, which receives the molten metal from a tundish and casts the metal into a generally thin strip. The strip may be further subjected to cooling and processing upon exit from the casting rolls.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontal casting rolls which are internally cooled so that metal shells solidify on the moving casting roll surfaces, and are brought together at the nip between the casting rolls to produce a thin cast strip product. The thin cast strip is delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip, to form a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the casting rolls so as to restrain the two ends of the casting pool.
When casting steel strip in a twin roll caster, the thin cast strip leaves the nip at very high temperatures, of the order of 1400° C. If exposed to normal atmosphere, it will suffer very rapid scaling due to oxidation at such high temperatures. A sealed enclosure that contains an atmosphere that inhibits oxidation of the strip is therefore provided beneath the casting rolls to receive the thin cast strip, and through which the strip passes away from the strip caster.
The length of a casting campaign of a twin roll caster has been generally determined in the past by the wear cycle on the core nozzle, tundish and side dams. Therefore, the focus of attention in the casting has been to extend the life cycle of the core nozzle, tundish and side dams, and thereby reducing the cost per ton of casting thin strip. When a nozzle, tundish or side dam wears to the point that one of them has to be replaced, the casting campaign has to be stopped, and the worn out component replaced. This generally involves replacing other unworn components as well, otherwise the length of the next campaign would be limited by the remaining useful life of the worn but not replaced refractory components. Graphite alumina, boron nitride and boron nitride-zirconia composites are examples of suitable refractory materials for the side dams, tundish and core nozzle components. Since the core nozzle, tundish and side dams all have to be preheated to very high temperatures approaching that of the molten steel, there is considerable waste of casting time between campaigns. See U.S. Pat. Nos. 5,184,668 and 5,277,243.
The side dams wear independently of the core nozzles and tundish, and independently of each other. During casting the side dams are initially urged against the ends of the casting rolls under applied forces, and “bedded in” by wear so as to ensure adequate seating against outflow of molten steel from the casting pool. The forces applied to the side dams are then reduced after an initial bedding-in period, however there is significant wear of the side dams throughout the casting operation. The core nozzle and tundish components in the metal delivery system usually have a longer potential life than the side dams, and could normally continue in service through several more ladles of molten steel if the useful life of the side dams could be extended. However, the tundish and core nozzle components, which still have useful life, are changed when the side dams are changed to increase the production capacity of the caster.
Previously, each side dam was generally held in place during casting by a side dam holder. The side dam typically included a V-shaped beveled bottom portion and the side dam holder typically included a V-shaped receptacle into which the V-shaped beveled bottom portion of the side dam was seated. The V-shape configuration served to position and hold the side dam in place during casting. However, such side dam assemblies limited the useful life of the side dams before causing serious damage to the casting equipment as well as adversely impacting the edges of the cast strip. Specifically, the degree of side dam wear had to be limited to prevent the clashing of the side dam holder V shaped receptacle with the casting roll edge, limiting the service life of the side dam. Therefore, the side dams were always replaced before such damage to casting equipment could occur, limiting the duration of the casting campaign. As explained above, when the side dams were changed, the removable tundish and core nozzle were generally also changed and a new casting campaign started. The casting costs per ton of thin strip cast thus could be considerably reduced if the useful life of the side dams could be extended.
In summary, no matter which refractory component has worn out first, a casting campaign will need to be terminated to replace the worn out component. Since the cost of thin cast strip production is directly related to the length of the casting time, unworn components in the metal delivery system are generally replaced before the end of their useful life as a precaution to avoid further disruption of the next casting campaign. This results in attendant waste of useful life of refractory components.
Further limitations and disadvantages of previously used and proposed thin strip casting systems and methods will become apparent to one of skill in the art, through comparison of such systems and methods with the present invention as set forth in this present application.
A method of producing thin cast strip by continuous casting is disclosed comprising the steps of:
a) assembling a pair of casting rolls having a nip therebetween,
b) assembling a metal delivery system comprising side dams adjacent the ends of the nip to confine a casting pool of molten metal supported on casting surfaces of the casting rolls, where each side dam has an upper portion adjacent a lower portion as described below,
c) providing force devices capable of independently urging said upper side dam portion and said lower side dam portion of each side dam toward the caster rolls during casting,
d) introducing molten steel between the pair of casting rolls to form a casting pool supported on casting surfaces of the casting rolls confined by the side dams, and
e) counter-rotating the casting rolls to form solidified metal shells on the surfaces of the casting rolls and cast thin steel strip through the nip between the casting rolls from the solidified shells.
Each lower side dam portion is assembled to have opposite outer surfaces, with one outer surface which is capable of contacting molten metal at the nip, and an opposite outer surface having a fastening portion capable of attaching the lower portion of the side dam to a lower side dam holder to hold the lower portions of the side dam in place during casting. Each lower side dam portion may be confined to the portion of the side dam of greater wear capable of contacting the molten metal adjacent the nip, and may be substantially thicker than the upper side dam portion. For example, the lower side dam portion may be about 30 mm, or more, in height. As a result, the useful life of the overall side dam can be effectively extended.
Alternatively, or in addition, the lower side dam portion may be substantially longer than needed for the operation of the caster, and may be supported to move laterally. By this arrangement, as the lower side dam portion wears, the part of the lower side dam portion capable of being in contact with the molten metal adjacent the nip can be changed by moving the lower portion of the side dam laterally without moving the upper portion of the side dam. Again, the useful life of the overall side dam can be extended by this arrangement.
However arranged, the lower side dam portion may be fastened by refractory fastener portions, extending beyond the outer surface of the lower side dam portion and interacting with attachments portions on a lower side dam holder to position the lower side dam portion. The fastening portions of each lower side dam portion may comprise ceramic pins which are attached into the opposite outer surface portion of each lower side dam portion. The fastening portions hold the lower side dam portions in place without a substantial exposed portion of the lower side dam holder extending beyond the opposite outer surface of the lower portion of the side dam
Each upper side dam portion is also assembled to have opposite outer surfaces, with one outer surface capable of contacting the molten metal and the opposite outer surface having fastening portions capable of attaching the upper portion of the side dam to an upper side dam holder to hold the upper portions of the side dams in place during casting. The fastening portions hold the upper side dam portions in place without a substantial exposed portion of the upper side dam holder extending beyond the opposite outer surface of the upper portion of the side dam, and without the upper side dam holder preventing a bottom surface of the upper portion of the side dam from being positioned adjacent to a top surface of the lower portion of the side dam. Each upper side dam portion also may have refractory fasteners extending beyond the opposite outer surface adjacent to a side dam holder. These refractory fasteners of each upper side dam portion and attachment portions of each side dam holder may interact to position the upper side dam portion during casting. The fastening portions of each upper side dam portion may comprise ceramic pins which are attached into the opposite outer surface portion of each upper side dam portion.
Each lower and upper side dam holder may have attachment portions comprising notches, or troughs, into which fastening portions of the lower or upper side dam portion can seat, when the lower or upper side dam portion is attached to the side dam holder for a casting campaign. Alternatively, the lower and upper side dam holders may have attachment portions, which are usually ceramic, that extend into the fastening portions of the lower or upper side dam portions (which are openings in the lower or upper side dam portion), so that the circumferentially exposed portions of the lower and upper side dam holder do not extend substantially beyond the opposite outer surface of the lower or upper side dam portion toward the outer surface capable of contacting the molten metal.
A continuous thin strip casting system is also disclosed with side dam assemblies at each side of the caster. Each side dam assembly comprises a lower side dam portion and an upper side dam portion each having opposite outer surfaces, where one outer surface is capable of contacting molten metal and the opposite outer surface has fastening portions capable of attaching the lower or upper side dam portion to a lower or upper side dam holder to hold the lower or upper side dam portion, respectively, toward the casting rolls during casting. The upper side dam portion and the lower side dam portion are supported independently of each other, and may be capable of being independently driven toward caster rolls of the twin roll caster system.
The side dam assembly further comprises lower and upper side dam holders having attachment portions capable of receiving and supporting the lower or upper side dam portion at the fastening portions, without any exposed portion of the side dam holders extending substantially beyond the opposite outer surfaces of the lower or upper side dam portions toward the outer surfaces capable of contacting molten metal.
The bottom surface of the upper side dam portion is adjacent a top surface of the lower side dam portion, and the lower side dam portion is thicker than the upper side dam portion. The thickness of the lower side dam portion may be at least 20% thicker than the upper side dam portion, and is typically limited to the area of the lower portion of a side dam that experiences the greater wear adjacent the nip. As a result, the thicker lower side dam portion extends the overall operational life of the side dam assembly either by being thicker in the greater wear areas of the side dam in contact with the casting pool, or by being laterally moveable so that new surface areas of the lower side dam portion can be exposed to molten metal as the casting campaign continues, or both, without removing the upper side dam portion.
The side dam assembly may comprise a lower side dam portion having at least one ceramic pin extending outward from the opposite outer surface capable of attaching to the attachment portions of the lower side dam holder and holding the lower side dam portion in place during casting. The lower side dam portion may be longer than needed for contacting the molten metal adjacent the nip and be positioned to move laterally, so that a different area of an outer surface of the lower side dam may be brought into position as the lower portion of the side dam wears during a casting campaign. The side dam assembly may also comprise an upper side dam portion having at least three ceramic pins extending outward from the opposite outer surface capable of attaching to the attachment portions of the side dam holder and holding the upper side dam portion in place during casting. The side dam assembly also may comprise lower and upper side dam holders having notches, or troughs, capable of positioning and supporting the lower or upper side dam portion during casting, without any exposed portion of the side dam holder extending substantially beyond the opposite outer surface of the upper side dam portion toward the surface portion of the lower or upper side dam portion capable of contacting molten metal, and without any portion of the side dam holder preventing a bottom surface of the upper side dam portion from being positioned adjacent to a top surface of a lower side dam portion.
The system and method of continuously casting thin strip, with the disclosed side dam assembly, can extend the length of a casting campaign by as much as 50% or more. The useful life of the side dams can be extended without damage to the casting equipment or risk of bleeding of molten metal from the casting pool damaging to the edges of the cast strip—resulting in termination of the casting sequence. Also, with certain embodiments of the present invention, the positioning of the side dams after preheating by robots is facilitated by assembling the side dams in place for casting, and with certain embodiments of the present invention, the positioning of the lower side dam portion can be done without preheating or without changing the upper side dam portion at the same time, or both.
These and other advantages and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings.